Corona discharges are used widely to promote adhesion between various materials. In manufacturing photographic products there is a large body of literature describing various applications of coronas to make aqueous and non-aqueous coatings adhere to various substrate materials. Almost all of these coronas are produced by applying a high voltage (approximately 5-10 kV), relatively high frequency (10 kHz) signal to electrodes in an ambient atmosphere at atmospheric pressure. See for example, U.S. Pat. Nos. 4,241,169; 4,701,403; 4,087,575; 4,429,032; 4,363,872; 4,229,523; 4,394,442; 3,411,908; 3,531,314; 3,582,339; 3,607,345; 3,630,742; 3,860,427; 3,874,877; 3,888,753; 4,055,685; 4,518,681; 5,004,669; French Patent 7613034 and European Patent Application No. 92303556.2. However, there are limitations to the usefulness of corona treatments. Corona treatments produce locally energetic discharges, commonly known as streamers, and these streamers may cause a non-uniform level of treatment. They may also be related to an inhomogeneous loss of red speed in photographic emulsions which produces a mottle defect. Furthermore, corona treatments appear to be effective at promoting adhesions of various coatings to polyethylene but are relatively ineffective at promoting the adhesion of layers to various polyester supports such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).
A more controllable and effective way of preparing polymers for coating is with a low pressure glow discharge treatment. Glow discharges are, by nature, very diffuse and homogeneous, producing a more uniform treatment. Moreover, by controlling the gas it is possible to improve the adhesion of photographic layers to materials such as polyesters as well as polyethylene. See for example, U.S. Pat. Nos. 4,993,267; 3,837,886 and 4,451,497. A major disadvantage in glow discharge treatments done at reduced pressures is the problem of maintaining a low pressure at the treatment station. It is necessary to use either a batch process, in which the support is loaded into a treatment chamber and the air is removed, or an in-line process which requires that the support pass through a differential pressure region. In the first case, the support must go through an additional off-line step before coatings are applied. This is unattractive from a product flow perspective and requires additional capital. The second choice is difficult and expensive to implement because of the very tight tolerances needed to maintain the pressure differentials in the transport path. This requires expensive and complicated hardware and pumps. The closer to atmospheric pressure that the treatments can be done, the simpler and less costly the process becomes.
It is known that under certain conditions, stable diffuse glow discharges can be produced at atmospheric pressures. Articles that discuss stable glow discharges are as follows: S. Kanazwa, M. Kogoma, T. Moriwaki and S. Okazaki, J. Phys. D: Appl. Phys 21 (1988), pp. 838-840; S. Kanazwa, M. Kogoma, S. Okazaki and T. Moriwaki, Nuclear Instruments and Methods in Physics Research, B37/38 (1989) pp. 842-845; T. Yokoyama, M. Kogoma, S. Kanazawa, T. Moriwaki and S. Okazaki, J. Phys. D: Appl. Phys. 23 (1990) pp. 374-377; T. Yokoyama, M. Kogoma, T. Moriwaki and So. Okazaki, J. Phys. D: Appl. Phys. 23 (1990) pp. 1125-1128 and A. Nagata, S. Takehiro, H. Sumi, M. Kogoma, S. Okazaki, and Y. Horikie, Proc. Jpn. Symp. Plasma Chem 2 (1989) pp. 109-112. Work in this area has been limited and directed primarily at etching of photoresist and deposition of materials. These articles indicate that a reliable method of producing diffuse glow discharges at atmospheric pressure is to use helium as the discharge gas. The work reported in the literature has been reproduced and found to be reliable. It has also been found that very small amounts of reactive gases, such as a few percent nitrogen or oxygen, will extinguish an atmospheric helium discharge. However, we have also found that by using trace amounts of reactive gases, stable discharges can be produced at very close to atmospheric pressure which are also able to dramatically improve the adhesion of photographic emulsions to difficult to coat materials such as polyethylene, PET and PEN.
The present invention allows one to treat polymeric surfaces with a stable atmospheric glow discharge so that adhesion of photographic emulsions is improved between the photographic emulsions and the polymeric materials.